The present invention relates to a method for treating contaminated soils and ground waters. In particular, the invention relates to treating soil that is contaminated with halogenated hydrocarbons, such as halogenated hydrocarbons in aqueous compositions.
Halogenated hydrocarbons, such as chlorinated hydrocarbons, are also known as chlorinated solvents (hereinafter collectively referred to as xe2x80x9cchlorinated solventsxe2x80x9d). Halogenated hydrocarbons have low flammability and are fairly stable, both chemically and biologically. They are commonly used in industry as chemical carriers and solvents, paint removers, and cleaners. The cleaning applications typically include metal degreasing, circuit board cleaning, metal parts cleaning, and dry cleaning. Chlorinated solvents are also used as intermediates in chemical manufacturing and as carrier solvents for pesticides and herbicides.
Chlorinated solvents are stable compounds, are relatively toxic at low levels, and many chlorinated solvents have been classified as suspected or confirmed carcinogens. Chlorinated solvents are among prevalent contaminants in ground water and soil because of their widespread use and stability. Ground waters and soils have become contaminated by chlorinated solvents from various sources. These sources include, but are not limited to, disposal facilities, chemical spills, and leaking underground storage tanks. Chlorinated solvents also may be released to the environment through the use, loss, or disposal of a neat liquid, and alternatively through the use or disposal of wash and rinse waters containing residual solvents.
Movement and dispersion of chlorinated solvents in the subsurface soils and ground waters vary depending on whether the solvents are released as a neat liquid or in a dissolved form. If released in a dissolved form, chlorinated solvent migration is governed largely by hydro-geological conditions and processes. The presence of solubilizing agents, such as soaps from wash waters, counteracts natural soil sorption-retardation mechanisms for chlorinated solvents, and enhances migration of the chlorinated solvents.
If chlorinated solvent is released as a neat liquid, the chlorinated solvent migrates through soil under the force of gravity. A portion of the chlorinated solvent is typically retained in soil pores. If sufficient chlorinated solvent is present in the soil, the soil pores become saturated. Additional chlorinated solvent continues to migrate in the soil until it encounters a physical barrier or a water table. If the chlorinated solvent encounters a water table, the chlorinated solvent disperses until it encounters, accumulates, and overcomes the water table""s capillary forces. At this point, the chlorinated solvent, which has a greater density than water, penetrates the water table""s surface. The chlorinated solvent migrates under the force of gravity until its amount has been diminished through sorption, or until the chlorinated solvent encounters an aquitard.
In recent years, soil and ground water contamination by chlorinated solvents has become an environmental problem. Chlorinated ethylenes, such as trichloroethylene (TCE), tetrachloroethylene (commonly known as perchloroethylene (PCE)), and chlorinated ethanes, such as 1,1,1-trichloroethane (TCA), which have been used as degreasing agents in a variety of industrial applications, pose environmental problems. Even though chlorinated degreasing agent use was curtailed in 1976, improper storage and uncontrolled disposal practices have resulted in contamination. Due to the high water solubility of chlorinated solvents, for example about 1100 mg/L TCE at 25xc2x0 C., chlorinated solvents are highly mobile in soils and aquifers, and should be removed before dispersing too far. Therefore, a treatment to remove chlorinated solvents from contaminated soil and ground water is needed.
A pump-and-treat method is a proposed treatment method removing contaminants from contaminated ground water. The treatment usually involves withdrawing contaminated water from a well, volatilizing the contaminants in an air stripping tower, and adsorbing vapor-phase contaminants into granular-activated-carbon (GAC). There are limitations to this pump-and-treat method. The method is relatively inefficient, and some sites can require treatment for extended periods of time.
Chlorinated solvents can be degraded into less harmful materials by a method commonly referred to as xe2x80x9creductive dechlorination,xe2x80x9d in which chlorine is replaced by hydrogen. The reductive dechlorination uses metallic, solid reaction elements, such as iron and zinc, to degrade chlorinated solvents and other organic compounds. For example, Gillham, U.S. Pat. No. 5,266,213, discloses feeding contaminated ground water through a trench containing iron to degrade contaminants. The Gillham process is conducted under strict exclusion of oxygen and occurs over a long time period. The Gillham process often requires large amounts of iron for complete reaction. Furthermore, it is difficult to introduce large volumes of solid reaction material, such as iron, using the Gillham process at effective depths for in situ remediation.
Clarke et al., U.S. Pat. No. 5,861,090, discloses a method that electrochemically remediates soil, clay, or other organic-polluted, contaminated media. The Clarke process remediates contaminated media using Fenton""s Reagent. In Clarke, anodes and cathodes are provided in wells, which are disposed in the contaminated media. Anolyte and catholyte solutions are circulated in the contaminated media to deliver ions, such as-iron ions, to anodes and to deliver ions, such as peroxide ions, to cathodes. A potential difference is applied across the contaminated media and causes the peroxide and iron ions to migrate toward each other through the contaminated media. The organic pollutants are destroyed by reactions with the ions. While Clarke teaches possible contaminated content monitoring and adjusting steps, Clarke does not disclose control of potential difference in response to contaminant content monitoring.
Therefore, a controllable process that effectively treats contaminated soils and ground waters compositions is needed, particularly for controlling a potential difference applied to the contaminated media. Further, the process should enable control of potential difference in response to contaminant content monitoring.
The present invention provides a method for treating contaminated media. The method comprises introducing remediating ions consisting essentially of ferrous ions, and being peroxide-free, in the contaminated media; applying a potential difference across the contaminated media to cause the remediating ions to migrate into contact with contaminants in the contaminated media; chemically degrading contaminants in the contaminated media by contact with the remediating ions; monitoring the contaminated media for degradation products of the contaminants; and controlling the step of applying the potential difference across the contaminated media in response to the step of monitoring.
In another embodiment of the present invention, a method for treating contaminated media comprises determining a chlorinated hydrocarbon content of the contaminated media by sampling and analysis; introducing remediating ions being peroxide free, at an electrode disposed proximate the contaminated media; applying a potential difference across the contaminated media between electrodes to cause the remediating ions to migrate into contact with chlorinated hydrocarbons in the contaminated soil region; chemically degrading contaminants in the contaminated media by contact with the remediating ions to produce chloride ions; determining a chloride ion content; and controlling the step of applying the potential difference, the step of controlling being in response to the chloride ion content.
In a further embodiment of the present invention, a method for treating a contaminated media includes the steps of introducing ferrous ions, said ions being peroxide-free, at an iron-containing anode disposed proximate the contaminated media; applying a potential difference across the contaminated media between at least one cathode and the iron-containing anode that are disposed proximate the contaminated media to cause the remediatitig ions to migrate into contact with contaminants in the contaminated media.